In many optical information storage and retrieval systems, a radiation beam from an optical source is reflected and diffracted from a data track on an optical storage medium. The beam returning from the storage medium may be directed to a detector array that provides signals used to generate, for example, a focus error signal (FES), a tracking error signal (TES), and a data signal. The FES and TES generally drive servo systems for maintaining the radiation beam in-focus and on-track, respectively, relative to the storage medium. The data signal is indicative of the data stored on the data track scanned by the radiation beam. The portion of the optical system which generates and processes the radiation beam is generally referred to as an optical head.
The stability of an optical head is usually improved by decreasing the distance between certain critical components, such as an optical source, beam splitter and detector array. In addition, the cost and complexity of the optical head is reduced if these components are integrated in to a single package. A known technique for accomplishing these objectives involves combining components such as an optical source, a grating beam splitter and a detector array into an integrated package generally referred to as a laser-detector-grating unit (LDGU). LDGUs are also known as laser/detector optical heads and hologram laser units. Optical systems which incorporate an LDGU or a similar device will be referred to herein as LDGU-based systems. A number of exemplary LDGU-based systems are described in W. Ophey, "Compact Optical Light Paths," Jpn. J. Appl. Phys., Vol. 32, Part 1, No. 11B, pp. 5252-5257, November 1993. Other LDGU-based systems are described in, for example, U.S. Pat. Nos. 5,050,153 and 4,945,529. An exemplary optical head in accordance with U.S. Pat. No. 4,945,529 includes a diffraction grating with four grating regions. The four grating regions direct portions of a reflected and diffracted radiation beam to a detector assembly in order to generate an FES, a TES and a data signal.
The above-noted LDGU-based systems suffer from a number of drawbacks. For example, the optical source is generally not sufficiently isolated from the return beam, resulting in increased optical source noise. The optical source noise may result from phenomena such as longitudinal mode-hopping. Existing LDGUs also typically have an inherently low throughput efficiency, due in part to the fact that the radiation beam is generally not circularized. A circularized radiation beam is rotationally symmetrical about its optical axis. Throughput efficiency may be defined in terms of a percentage of optical source radiation which is transferred to the surface of the optical storage medium. Currently available LDGUs used for reading optical disks have throughput efficiencies on the order of only about 10%, with a consider able amount of the optical source output lost in the grating beam splitter and in truncating the non-circularized radiation beam. Although LDGUs are now commonly used for read-only applications such as compact disk (CD) players, the problems of source noise and low throughput efficiency have limited the usefulness of LDGUs in higher power applications such as optical recording.
In addition, some LDGU designs exhibit excessive optical crosstalk between tracking and focus signals. The optical crosstalk originates from, for example, diffracted radiation components and optical wavefront aberrations in the return beam. The presence of optical crosstalk may limit the effectiveness of LDGUs in certain optical systems, particularly those systems which utilize high performance focus and tracking servomechanisms. Although U.S. Pat. No. 5,406,541 reduces the effect of crosstalk in optical heads by implementing an orthogonality condition between the focus and tracking sensors, it does so by using separate optical paths for generating the focus and tracking signals. The need for additional components to create and process separate optical paths adversely affects the cost and complexity of the optical head.
Furthermore, the previously mentioned LDGU designs do not permit the generation of a differential phase tracking error signal.